cmd/go/internal/lockedfile/internal/filelock: use errors.ErrUnsupported

cmd/go/internal/lockedfile/internal/filelock: use errors.ErrUnsupported

All platform specific errors are now covered by errors.ErrUnsupported.

Updates #41198

Change-Id: Ia9c0cad7c493305835bd5a1f349446cec409f686
Reviewed-on: https://go-review.googlesource.com/c/go/+/476917
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>